(Recasts with Petraeus comments)

BAGHDAD, May 19 (Reuters) - The U.S. military's top commander in Iraq said he believed at least two of three missing U.S. soldiers abducted a week ago were still alive, while a massive, week-long search widened.

Thousands of U.S. and Iraqi troops have been scouring farmlands through an area south of Baghdad known as the "triangle of death" since an ambush a week ago in which four U.S. soldiers and an Iraqi translator were also killed.

General David Petraeus, the commander of U.S. forces in Iraq, said it had been feared one of the missing three might have died, but he did not know for certain if all three were alive or dead.

"As of this morning, we thought there were at least two that were probably still alive," Petraeus told the U.S. Army newspaper Army Times in an interview conducted on Friday.

"At one point in time there was a sense that one of them might have died, but again we just don't know," he said.

The al Qaeda-led Islamic State in Iraq has mocked U.S. forces searching for their comrades and warned them to call off the search in order to guarantee their safety.

The Sunni Arab militant group has not given any proof that the soldiers are alive or that it has them and has not issued any demands for their release.

Petraeus said searchers knew who was behind the attack, one of the worst against U.S. forces since the 2003 invasion.

"He's sort of an affiliate of al Qaeda," Petraeus said.

"He's the big player down in that area. We've tangled with him before," he said.

The U.S. military in Iraq earlier said nine people suspected of involvement had been detained during a raid on a building in Amiriya, a Sunni Arab stronghold 40 km (25 miles) west of Baghdad, early on Saturday.

The search had previously been focused in and around Mahmudiya, where the attack occurred last Saturday, and nearby Yusufiya, south of Baghdad.

Spokesman Lieutenant-Colonel Christopher Garver said more than 700 people had been held for "tactical questioning" since the search began. Of 11 people detained before Saturday, four had been identified as "high-value targets", Garver said. "Obviously if they're persons of interest we put them in the detention system," Garver said. The soldiers, part of a larger unit sent to intercept roadside bombers in a Sunni Arab militant stronghold, went missing after an apparently coordinated ambush.

Evidence collected at the scene suggested the missing soldiers had put up a fight before they were captured, the military has said.

U.S. President George W. Bush has sent thousands of extra troops into Baghdad and other areas in a last-ditch bid to avoid all-out civil war between Sunni Arabs, dominant under Saddam Hussein, and majority Shi'ites.

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